Before they can proceed through the secretory pathway, immunoglobulin L and H chains require assembly in the Endoplasmic Reticulum. At least one element of this secretory requirement is an epitope on the solvent-exposed surface of the VL domain. Mutations in this epitope block secretion without impairing the ability of the subunits to assemble into a functional antibody. Mutant Ig are arrested in the ER, but are not associated with BiP, indicating the existence of another protein interaction which is necessary for secretion. The relevant VL amino acids are conserved among several other proteins, each of which also requires subunit assembly before it can be expressed on the cell surface. All of these proteins are important either for the development of T and B lymphocytes or for their antigen recognition. The non-secreted mutants will be used to characterize protein-protein interactions that are essential for proper secretion of Ig and related proteins. First, the VL secretion epitope will be mapped more precisely, and its effects on the structure and folding of L chains will be defined. Second, the mode of action of the secretion epitope will be tested, to determine if it has properties expected from a ligand for an ER receptor, and if it is both necessary and sufficient for secretion. Third, the prediction that a similar mechanism controls the surface expression of other proteins will be tested by mutating the VpreB subunits of the pre-B cell "receptor", and assessing the intracellular fate of the mutant proteins. Pre-B cells expressing the mutated "receptor" will be used to probe its yet unknown function. Fourth, a different structural element in the CH1 domain, which causes retention of unassembled H chains (but not of tetrameric Ig) will be mapped and its mode of action in relation to the L chain epitope will be determined. Fifth, biochemical and genetic approaches will be used to identify and characterize the putative ER protein(s) which recognizes the VL secretion epitope. Finally, two other protein interactions which occur before Ig assembly will be characterized -the association of Ig subunits with BiP and with another ER stress protein, GRP94. Together with the protein that recognizes the secretion epitope, BiP and GRP94 may comprise a quality control system which allows traffic from the ER to the Golgi. By describing new control steps in the biosynthesis and secretion of Ig, these studies will explain the relation between its folding, assembly and proper traffic in the cell. These control steps may be shared by several Ig-related molecules, regulating their surface expression. The structural requirements for secretion represent a selective force which shapes V gene repertoires of lymphocytes, and if not satisfied may underlie several types of B and T cell immunodeficiencies.